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Monday, August 18, 2014

Why Is Washington State Experiencing More Wildfires than Southern/Central California?

The drought in California has been given extensive coverage in the media, with stories last spring predicting a heavy toll from major wildfires over the summer. This has not come to pass.

After a few early wildfires near San Diego in April, California has seen few major wildfires, but Washington State has been hit very hard, with many major fires in the Cascades and eastern Washington (see graphic). Eight major fires are now burning over Washington (red circles), twice the number found in California.

Looking at the U.S. Drought Monitor diagnostics (which show the moisture content of the surface layer), you would think that California would be FIRE CENTRAL, with FAR drier conditions than in Washington (I show the maps for August 12th and July 1st).

But that has not been the case. So why are so many fires occurring in Washington while California has been spared the worst so far?

As I will explain below, the initiation of wildfires is a complicated issue, with far more involved than drought and a dry surface. A number of factors need to come together to get major fires, and Washington (and particularly eastern WA) has had them. These situation also has implications for what might happen under global warming, as the Southwest dries out.

The situation over the Northwest has been ideal for fires this summer. We have had long periods of warm, dry conditions that has dried out the surface and caused relative humidities to plummet. And then weak troughs have approached us, covering the area with lots of lightning that have initiated fires.

Consider Wenatchee, on the eastern slopes of the Cascades and close to several major fires. Below you can see the temperatures, relative humidity, and temperature. In mid July temperatures rose into the 90s and low 100s F, with relative humidities dropping below 15%. And then rain and thunderstorms hit around July 23, setting off a series of fires. Then the warming started again and the humidity plummeted. And then ran and thunderstorms hit again last week, causing more fires.

Each of the lightning events was caused by the approach of an upper level trough, like the one shown below, that interrupted a warm, high pressure regime. The passage of the trough was associated with strong winds on the eastern slopes that stoked the fires, sometimes causing them to explode into uncontrollable infernos.

So for us in the Northwest, we have had perfect conditions for fires...long warm, dry periods interspersed with brief periods of thunderstorms.

By why has California lucked out? First, other than April they have had a general absence of dry, Santa Ana conditions in which strong, offshore flow brings extremely low humidities, heat, and powerful winds rev up fires.

They have not had much lightning because the troughs moved in over the Northwest and not over southern and central CA.

And they have actually had quite a bit of precipitation over the Sierras, with a stronger than normal wet Southwest Monsoon. To show this, here is the percent of normal precipitation over the western U.S. for the last 60 days. The blues and purples are well above average.

The summer is not over yet, and the great Santa Ana fires generally hit California in the fall, so California is not out of the woods yet. But the big message is that the initiation of major wildfires is a complicated affair, and simplistic arguments about drought and fire can be pushed too far if we are not careful.

2 comments:

Unfortunately for the State of Jefferson, Siskiyou and Tehama Counties have been making up for the rest of CA. . .pushing 150,000 acres burned so far. Not as big as the Carlton Complex, but noteworthy.

October seems to be the height of Santa Ana wind season for central, and southwestern California. They may luck out this year if the El nino conditions kick in in time. Although I believe that this El nino will be a dud, it may be enough to help out California before it fizzles.